This invention relates to a permanent magnet rotary type electrical device or machine such as an electric motor or generator and to an improved method for making such machines.
As is known, rotary electric machines such as a brushless DC motor or generator utilize a permanent magnet element having a plurality of permanent magnets fixed thereon. The permanent magnet element is juxtaposed to an element having a plurality of poles around which coils are wound and the elements are supported for relative rotation. In a motor, the magnetic field formed by the coil windings is controlled so as to effect rotation. In a generator, relative rotation induces voltage in coil windings.
In such machines, the permanent magnets are disposed at circumferentially equal intervals. The slots between the core teeth are also disposed circumferentially at equal intervals. When the machine is used as a generator, it is desirable that the torque necessary to effect rotatation when output of the generator is zero (referred to as a cogging torque) is as small as possible. Increased cogging torque will necessitates a larger driving force of an engine or the like to rotate the generator, thus resulting in a larger vibration and load. If it is used as a motor, drive torque in idling becomes larger resulting in power loss as well as increased.
It is generally accepted that the cogging torque is inversely proportional to the square of what is referred to as a cogging number. This cogging number is basically the number of coggings that occur each revolution of the machine. This being the number of times the magnetic cores pole passes a permanent magnet per revolution.
Therefore, one prior art method used to decrease the cogging torque or the variation in rotation is to increase the number of poles. For example, a motor or a generator in which the number of slots S between the poles on which the coils are wound is nine and the number of magnetic poles P is eight, that is, a system of 9-slot and 8-pole construction, is known. In this case, the cogging frequency or cogging number has a sufficiently high value of 72/revolution. Therefore, the cogging torque becomes sufficiently small.
With this method, however, it is necessary to increase the number of poles P and the number of slots S, and the construction becomes more complex with increasing P or S, causing an increase in the number of parts as well as making the assembly more expensive to produce.
Another prior art method for reducing the cogging torque employs permanent magnets or slots that are disposed oblique to the rotational axis (that is, a so-called xe2x80x9cskewxe2x80x9d is imparted). This method is employed to smoothen the circumferential distribution change in the magnetic field so as to decrease the cogging torque.
In this method, permanent magnets of special shape must be made, which increases manufacturing costs and is not suited for the automation of the production and assembly.
Therefore it is a principal object of this invention to provide a permanent magnet type rotary electric device and method of manufacture therefore capable of effecting decreased cogging torque and suited for automation of manufacturing without increasing cost.
A first feature of this invention is adapted to be embodied in a permanent magnet type rotary electric machine of reduced cogging torque. The machine has a permanent magnet element comprised of a plurality of permanent magnets of alternate polarity in a circumferential direction. These permanent magnets are of substantially the same shape and disposed circumferentially at equal intervals. An armature element is juxtaposed to the permanent magnet element and has a number of circumferentially spaced core teeth around which coils are wound. The core teeth defining a number N of slots therebetween. The permanent magnet element and the armature element are supported for relative rotation. The circumferential magnet angle made by the permanent magnets with respect to the axis of relative rotation produces an effective cogging number that is substantially greater than the least common multiple of the number of slots S and the number of poles P.
A second feature of the invention is adapted to be embodied in a permanent magnet type rotary electric machine of the type described in the preceding paragraph. In accordance with this feature, the magnitudes of torque exerted on the permanent magnets of N and S poles of the magnet element are selected so that both of the torque values substantially cancel each other.
Yet another feature of the invention is adapted to be embodied in a method of making a permanent magnet type rotary electric machine of reduced cogging torque. The method comprising the steps of constructing a permanent magnet element comprised of a plurality of permanent magnets of alternate polarity in a circumferential direction with the permanent magnets being of substantially the same shape and disposed circumferentially at equal intervals. An armature element is juxtaposed to the permanent magnet element and has a number of circumferentially spaced core teeth around which coils are wound. The core teeth define a number N of slots therebetween. The permanent magnet element and the armature element are supported for relative rotation. The circumferential magnet angle made by the permanent magnets with respect to the axis of relative rotation is selected to produce a cogging number that is substantially greater than the least common multiple of the number of slots S and the number of poles P.
A still further feature of the invention is adapted to be embodied in a method as set forth in the preceding paragraph, however the magnitudes of torque exerted on the permanent magnets of N and S poles of the magnet element are selected so that both of the torque values substantially cancel each other.